This invention relates to a velocity modulation tube comprising a plurality of intermediate or floating resonators and short drift spaces. The floating resonator of a velocity modulation tube is defined as a resonator which is placed intermediate the input and the output resonators and which has neither a source of energy external to the tube nor a load external to the tube for utilizing the output power of the resonator. This, however, does not preclude a circuit element coupled to the resonator solely for effecting certain characteristics of the resonator.
In connection with a velocity modulation tube, it should be noted here that the interaction gap associated with a resonator contributes to the interaction between the electron beam produced in the tube and the electromagnetic field induced in the resonator and that the drift space interposed between two adjacent interaction gaps contributes to bunching of the velocity modulated electron beam. The length of a drift space is generally expressed by the normalized length in terms of the reduced plasma angle given by .omega..sub.q 1/u.sub.o, where .omega..sub.q represents the reduced plasma angular velocity, u.sub.o represents the D.C. beam speed, and 1 represents the physical distance between the middle points of the interaction gaps placed at both ends of the drift space. For brevity, the phrase "in terms of the reduced plasma angle" will be omitted in the following where possible. In addition, a drift space located immediately downstream of the interaction gap associated with a resonator will merely be referred to as a drift space located or placed immediately downstream of the resonator where intelligible. The words "upstream" and "downstream" refer to the macroscopic flow of the electron bean.
In a multi-cavity velocity modulation tube, it is usual to select various frequencies for the fundamental modes of resonance of the resonators in order to improve the gain-to-frequency characteristics of the tube. Thus, it has been conventional to tune the input resonator to the center frequency of the operating passband of the tube or to a somewhat higher frequency and to tune a floating prebuncher placed immediately downstream of the input resonator to a frequency lower than the center frequency. Consequently, the voltage induced across the gap of the floating prebuncher in the adjacency of the center frequency is nearly in phase opposition to the voltage produced across the gap of the input resonator to debunch the once bunched electron beam. If the drift space placed immediately downstream of the floating prebuncher is shorter than 60.degree., there is no room for the debunched electron beam to again become bunched. This prevents the electrons from being desirably bunched at the gap of the output resonator to result in a defect of limited conversion efficiency of the tube.
In order to raise the conversion efficiency of velocity modulation tubes, it has been proposed by Erling L. Lien in his U.S. Pat. No. 3,622,834 to use between a floating prebuncher and a final prebuncher placed next downstream of the floating prebuncher a lengthy drift space whose normalized length is between 90.degree. and 150.degree. , preferably 120.degree.. The tube, however, is difficult to handle due to its abnormal length and renders a UHF television transmitter bulky. In addition, the operable or tunable frequency range of the tube is narrow and restricted because use is made of the second harmonic space charge force and because the long drift sapce must be about 120.degree. long at the operating frequency.
On the other hand, a velocity modulation tube comprising drift spaces whose normalized lengths are only 90.degree. or less is revealed in U.S. Pat. No. 3,819,977 issued to Takao Kageyama, one of the present joint applicants, wherein the drift space disposed immediately downstream of the floating prebuncher is made longer than other drift spaces but not longer than 90.degree.. This tube is advantageous in that the operable frequency range or band is relatively wide because no use is made of the second harmonic space charge force. It is, however, to be noted that the reduced plasma angle becomes long to make the normalized drift length of the drift space in question shorter than 60.degree. in case a tube designed for higher frequency channels of, for example, a UHF television transmitter, is used for lower frequency channels with the resonance frequencies of the resonators merely reduced. This makes it impossible for the debunched electron beam to be bunched anew in the drift space and reduces the conversion efficiency.
In U.S. Pat. No. 3,725,721 issued to Martin E. Levin, a velocity modulation tube of a wide operable or tunable frequency band is disclosed, wherein a floating prebuncher and a final prebuncher placed next downstream of the floating prebuncher are tuned to the low and high frequency ends of the operating passband of the tube, respectively, as in a prior art velocity modulation tube. According to this patent, at least each of these prebunchers is provided with a series resonant circuit whose series resonant frequency is set at a frequency lower than the tunable band so as to automatically increase the Q-value of the relevant prebuncher to an optimum value with an increase in the frequency within the tunable band. This precludes the necessity of separate adjustment of the Q-values of these prebunchers but necessitates a somewhat complicated structure of such a prebuncher.